Cross-polarization isolation in satellite communication systems has becomes especially important in recent times, as an increasing number of systems implement the use of multiple signal polarizations (e.g., vertical and horizontal) to expand their communication capacity. Unfortunately, the cross polarization component of the signal will cause interference between the orthogonally polarized channels, e.g., between vertical and horizontal polarized channels. Minimization of this type of signal degradation is therefore desirable. The main source of cross polarization is in the feed system, namely the orthomode transducer (OMT). Accordingly, reducing the cross polarization in the OMT is an important task in ground systems design.
The source of the cross polarization component is the coupling via higher order modes in the OMT junction. Although those higher order modes are cut-off modes, they still act as a bridge to couple vertical and horizontal polarizations. A design with fewer higher order modes produced will result in less cross polarization coupling and, higher cross polarization isolation.
In practice, an OMT in the feed network is usually designed with a transmit reject filter (TRF) at the Rx port to reject the Tx signal at the Rx channel. It is preferable that the TRF be positioned close to the junction for easier OMT matching. The TRF may be designed as a band reject filter with multiple cavities. U.S. Pat. No. 5,739,734 entitled “Evanescent Mode Band Reject Filters and Related Methods” discloses an exemplary design of such filters.
FIG. 1A illustrates a conventional three-port OMT device 100. The three-port OMT includes a first waveguide section 120 coupled between an antenna (Ant) port 112 and a transmit (Tx) port 122, and a second waveguide section 130 coupled between the Ant port 112 and a receive (Rx) port 132. The TRF 134 is disposed within the second waveguide section 130. The common junction of the three port device is an asymmetrical junction, and it will produce many higher order modes.
FIG. 1B illustrates the conventional three-port OMT separated into half sections 100a and 100b, as shown. The aforementioned features are shown as having “a” and “b” counterparts, corresponding to half a or half b of the OMT 100, as shown. The TRF half 134a includes cavities for rejecting the Tx signal propagating within the Rx waveguide section 130.
FIG. 2A illustrates a conventional four-port OMT with symmetrical junctions 200. In this form, the four-port OMT 200 includes a first waveguide section 220 coupled between the Ant port 212 and a Tx port 222, a second waveguide section 230 coupled between the Ant port 212 and a first Rx port 232, and a third waveguide section 240 coupled between the Ant port 212 and a second Rx port 242. Conventionally, the second and third waveguides 230 and 240 are symmetrical, including implementation of matching TRF structures 234 and 244 in waveguide sections 230 and 240, respectively.
FIG. 2B illustrates the OMT 200 separated into half sections 200a and 200b, as shown. The aforementioned features are shown as having “a” and “b” counterparts, corresponding to half a or half b of the OMT 200, as shown. The TRF halves 234a and 234a include cavities for rejecting the Tx signal propagating within each of the Rx waveguide sections 240 and 250.
The four port OMT 200 excites fewer higher order modes due to the symmetry of its structure. Consequently, higher cross polarization isolation can be achieved. Disadvantageously, the two Rx ports 232 and 242 will require signal combining with a power combiner (not shown) to receive all components of the signal from satellite without any signal loss. An example of such a structure is disclosed by Wollack, E., in “A Full Waveguide Band Orthomode Junction” 1996 NRAO, EDIR Meme Series, #303. However, the overall network is cumbersome, and is not suitable to be used in a practical feed network.
Accordingly, what is needed is an orthomode transducer having improved cross polarization isolation similar to that provided by a larger symmetrical OMT, but which is of a compact size similar to the asymmetrical OMT.